1. Field of the Invention
The present invention relates in general to an image position matching method and apparatus therefor, and in particular to an improved method and apparatus for matching the positions between two images obtained of the same subject.
2. Description of the Related Art
It is a common practice in a wide variety of fields to comparatively read two or more images obtained of the same subject in order to discern the difference between the images, and then carry out an examination of the subject based on the thus discerned difference.
In the industrial products manufacturing field, for example, images obtained of a product when it is new and images obtained of the same product after it has been subjected to an endurance test are comparatively read, and attention is focused on the area showing the biggest difference so that an examination as to whether or not there are areas requiring improvement in product endurance can be carried out. Further, in the medical field, physicians comparatively read a plurality of radiation images of the diseased part of a patient, which have been obtained in a temporal series, in order to ascertain the progression of the disease and determine the proper course of treatment.
In this fashion, comparative reading of a plurality of images is carried out daily in a wide variety of fields. In general, two or more images are outputted as visual images by a display apparatus, a printer or the like, and these outputted visual images are arranged next to each other and comparatively read. At this time, the area of the most interest to the observer of such images is the area wherein the greatest difference is present, However, the proficiency in discerning the difference between the images in a comparative reading differs depending on the skill level and experience of the examiner; moreover, the smaller said difference is, the more difficult it becomes to discern. Therefore, there are calls for an improvement, which does not rely on the skill level of the examiner, in the performance of comparative reading.
In general, an interimage process, starting with a subtraction process performed between the structural positions (anatomically characteristic positions) of the two images that are the objects of comparative reading, is performed, and the difference between the images (hereinafter referred to as the interimage difference) is extracted and then enhanced (see Japanese Patent Application No. 11(1999)-342900). In this fashion, by extracting and enhancing only the interimage difference, because it is thereby made possible for a reader thereof to accurately recognize the interimage difference, it can be considered an effective means of preventing the oversight of a diseased portion or the progression of a disease.
Further, when this interimage process is performed, it is necessary that the respective position of each structural element (the structural positions) appearing in each image be correlated and matched. As to this position matching technology, there are two-step position matching technologies, for example, wherein: the positions of the entirety of two images are matched by moving the two images in parallel, rotating, or magnifying the images (e.g., a linear position matching employing an affine transform); a plurality of regions of interest (template regions) are set in one of the two images that has been subjected to the global position matching; a search region, which is larger than the template region, corresponding to each template region is set in the other of the aforementioned two images; a portion region (a corresponding template region), which is within the search region and which substantially matches the image appearing within said corresponding template region, is obtained for each group formed of a template region and the search region corresponding thereto; a shift quantity for matching the template region occurring in said one of said two images to the corresponding template region in the other of said two images is obtained, based on the positional relation between each template region occurring in one of said two images to each corresponding template region in the other of said two images; and a local position matching process is performed, by use of a non-linear transform (warping) employing a curve fitting process (a two dimensional n polynomial, wherein n≧2) on the two images of which the structural positions thereof have been matched (see Japanese Unexamined Patent Publication No. 7(1996)-37074, U.S. Pat. No. 5,982,915, etc.). According to these two-step position matching technologies, the structural positions occurring in two images can be matched comparatively favorably.
However, in images such as radiation images of the rib cage of a human patient, for example, in which bone tissues such as the ribs, vertebrae and the like, and the soft tissues such as muscle tissue, organs and the like are contained in the same image, there are cases in which, corresponding to changes in the orientation (standing position, inclination, etc.) of the subject of photographing (a human body) during the photographing thereof, the positional orientation of the bone tissue structures and the soft tissue structures shown in the images is different. Conventionally, for cases in which the bone tissue structures and the soft tissue structures shown in two images are displaced in different directions, a position matching process in which the positions of the entirety of the subject images are matched, such as that described above, is performed utilizing an original image (or a reduction image of the original image, a blurred image of the original image, etc.), whereby an image in which the positions of the soft tissue structures have been globally matched. However, if a local position matching process such as that described above is performed utilizing an image in which the positions of the soft tissue structures have been globally matched, there are cases in which ribs appearing in a template region do not appear in the corresponding search area; as a result, artifacts caused by an extraordinarily large positional misalignment between the ribs have appeared in the subtraction image (an image obtained by extracting and enhancing the difference between two images), which is formed after the local position matching process, which is the index process by which the positions of the ribs are matched, has been performed.
Meanwhile, because it is generally desirable that an image in which the positions of the soft tissue structures have been accurately matched be used for extracting a diseased portion present in the soft tissue structures, and that an image in which the bone tissue structures have been accurately matched be used for extracting a diseased portion present in the bone tissue structures, depending on the diagnostic objective, there are cases for which it is preferable, even if artifacts of the ribs (bone tissue structures) appear in a subtraction image, that the global matching be performed on the soft tissue structures.